linux-kernel - Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

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Message-ID: <4CFD16FC.8040905@redhat.com>
Date:	Mon, 06 Dec 2010 19:01:48 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Jan Kiszka <jan.kiszka@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Tom Lyon <pugs@...co.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI
 2.3 devices

On 12/06/2010 06:46 PM, Jan Kiszka wrote:
> Am 06.12.2010 17:40, Avi Kivity wrote:
> >  On 12/06/2010 06:34 PM, Jan Kiszka wrote:
> >>>
> >>>   What's the protocol for doing this?  I suppose userspace has to disable
> >>>   interrupts, ioctl(SET_INTX_MASK, masked), ..., ioctl(SET_INTX_MASK,
> >>>   unmasked), enable interrupts?
> >>
> >>  Userspace just has to synchronize against itself - what it already does:
> >>  qemu_mutex, and masking/unmasking is synchronous /wrt the the executing
> >>  VCPU. Otherwise, masking/unmasking is naturally racy, also in Real Life.
> >>  The guest resolves the remaining races.
> >
> >  I meant when qemu sets INTX_MASK and the kernel clears it immediately
> >  afterwards because the two are not synchronized.  I guess that won't
> >  happen in practice because playing with INTX_MASK is very rare.
>
> Ah, there is indeed a race, and the qemu-kvm patches I did not post yet
> (to wait for the kernel interface to settle) actually suffer from it:
> userspace needs to set the kernel mask before writing the config space
> (it's the other way around ATM). This avoids that the kernel overwrites
> what userspace just wrote out. We always suffer from the race the other
> way around, see below.

Please document the protocol.  Is this always the right order?  
Shouldn't it be reversed when unmasking?  I admit I'm confused about this.

> >>
> >>  I think this is what VFIO does and is surely cleaner than this approach.
> >>  But it's not possible with the existing interface (sysfs + KVM ioctls) -
> >>  or can you restrict the sysfs access to the config space in such details?
> >
> >  I'm sure you can, not sure it's worth it.  Can the situation be
> >  exploited?  what if userspace lies?
>
> That's also the above scenario inverted: Userspace can mask or unmask at
> any time. If it unmasks a yet unhandled, thus raise interrupt, it will
> trigger another one. The kernel will catch it and mask it again. That
> can repeat forever with the frequency userspace is able to run its
> unmasking code. Not nice, but nothing to leverage for a DoS.

Ok (I think).

-- 
error compiling committee.c: too many arguments to function

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